The use of optical fiber systems permits broad-band signals to be transmitted reliably and cheaply. In present communication networks, routing, switching and other signal processing operations are still carried out electronically, so that optical signals still must be converted to electrical signals at each end of the communication link. There is a growing need to postpone the electronic interface beyond the communication link and to perform signal processing operations in the optical domain.
Three classes of switches have been investigated in the optical domain: time division, wavelength division, and space division switches. In prior art optical time division switches (also known as time-slot interchangers), switching is carried out with architectures similar to their electronic counterparts. A time-multiplexed optical data stream is sequentially demultiplexed; the data in each time-slot is written into an optical memory cell; the contents of each cell is read out in a desired order and multiplexed onto an output highway thus accomplishing the desired time-slot permutation. Optical implementations of time division switches have used integrated optic switch matrices as optical write and read gates; and fiber optic delay lines or bistable laser diodes as optical memories. These approaches evidence a number of shortcomings, not the least of which being the requirement for large numbers of discrete devices to implement portions of the switching structures.
Wavelength-division switches will have the benefit of a potentially high throughput, but since only experimental wavelength converters are available, their development is at an early stage. Such a switch is described in "Multi-Wavelength Optical Cross Connect for Parallel-Processing Computers" by Arthurs et al., Electronic Letters, Vol. 24 (2) pp. 119-120, June 21, 1988. Therein is described a multiwavelength optical interconnection switch which employs a star coupler as a common optical cross-connect. Tunable laser transmitters provide various frequencies that are combined in the star coupler; transmitted in tandem to a plurality of fixed-wavelength receivers; with each receiver extracting its particular wavelength imposed. Laser transmitters which are economical and rapidly tunable over a wide range are not readily available. Thus, while a wavelength division system has the advantage of employing common transmission media, its method of encoding is not now economically practical.
Space division photonic switches commonly employ 4-port switching elements. The switching element that has received the most attention employs a Lithium Niobate, electro-optic crystal. Photonic switch arrays have been implemented by connecting several of these switching elements as optical cross points to provide physical paths for each signal. The fabrication of large optical switches using integrated optical switching elements is limited by several factors. First, the minimum cross sectional dimension required to confine light to a wave guide is approximately equal to its wavelength. Furthermore, the minimum interaction length required to produce switching is determined, in part, by the electro-optic strength of the material. Together, these factors restrict the minimum size of the optical switching element. The density of components is further limited by the minimum wave guide bend radius which exhibits acceptable loss. Finally, cumulative effects of wave guide propagation losses and crosstalk can be unacceptably high for large arrays.
Of the above-described optical switches, space division switching has seen substantial development. One subclass of space division switches (referred to as "time-multiplexed space division switches") employs a multiple access scheme wherein data is sent to all destinations, with each destination recognizing only that data which is directed to it. In articles entitled "Ultra Fast All-Optical Synchronous Multiple Access Fiber Networks" by Prucnal et al., IEEE Journal on Selected Areas in Communications, Vol. SAC-4 (9) December, (1986) pp. 1484-1493, and "TDMA Fiber-Optic Network with Optical Processing", Prucnal et al. Electronic Letters, Vol. 22 (23) 1986, pp. 1218-1219, both time division and code division, multiple access optical systems are described. In the time division multiple access system, each destination is assigned a time-slot during which the destination can receive a bit of information. All information is transmitted over a common transmission path and the destination only is enabled to receive information during its assigned time-slot.
In the code division multiple access system, a code sequence is transmitted and corresponds to the destination address. The destination recognizes the code sequence and uniquely recognizes that the data so encoded is destined for it. The code division multiple access scheme overcomes the time division system problem of having no data in an assigned time slot with a resultant loss of data transfer capability. However, the generation of multiple pulse addresses presents complexities in optical encoding and decoding system which are difficult to overcome.
It is known that the integration of large arrays of optical cross points are difficult to fabricate. If such an array network could be "collapsed" into a single switch configuration, both operational and cost benefits would accrue.
Accordingly, it is an object of this invention to provide a multiple access, optical communication system which employs a common transmission medium.
It is another object of this invention to provide a multiple access system wherein the encoding technique is particularly adapted to optical processing.